Heretofore, a refractory containing zirconia (ZrO2) as the main component, has been widely used at portions of inner walls of glass-melting furnaces which are in contact with molten glass, since it shows excellent corrosion resistance against molten glass.
However, a highly zirconia-based cast refractory composed mostly of ZrO2 crystals (baddeleyite) undergoes a reversible crystalline modification from monocline phase to tetragonal phase specific to the ZrO2 crystals in the vicinity of 1,100° C. and, hence, has a problem that due to abnormal volume expansion and shrinkage along with the crystalline modification, cracking is likely to occur particularly in the case of a refractory with a practical large size.
As a method for producing a refractory containing about 90 or higher mass % of ZrO2 which is free from such cracking, a method is known wherein to a glass phase (hereinafter referred to as matrix glass) composed mainly of SiO2 filling spaces among the ZrO2 crystals, a component to soften glass is incorporated to adjust the glass phase, so that a distortion due to the expansion and shrinkage of ZrO2 crystals within a temperature range for the crystalline modification of ZrO2 crystals, is absorbed by the soft matrix glass.
In such a case, it is common to use SiO2 as the main component of the matrix glass, but with SiO2 only, the viscosity will be high and it is difficult to absorb the abnormal volume change, and therefore, an alkali metal component (such as Na2O or K2O) or an alkaline earth metal component (such as CaO, MgO, SrO or BaO) is incorporated as a component to reduce the viscosity of the matrix glass. Such a component imparts to the matrix glass such a proper viscosity that the stress formed in a refractory in the transition temperature range of the baddeleyte crystals, can be relaxed. With the highly zirconia-based refractory thus obtained, cracking is reduced, and it can be used in a stabilized state for a long period of time.
On the other hand, in recent years, high purity glass or fine glass having little content of an alkali metal component and having a high melting point, has been used as glass for liquid crystal, and a highly zirconia-based refractory has now been used also for a glass-melting furnace to produce such glass.
However, when used as a refractory for lining a glass-melting furnace to melt low alkali glass, such a highly zirconia-based refractory is likely to have such a problem that the alkali metal component (mainly Na2O) tends to elute into the glass, and cracks are thereby likely to form in the refractory. That is, the alkali metal component contained in the matrix glass not only has an effect to lower the viscosity of the matrix glass but also has a function to suppress formation of zircon (ZrO2.SiO2) crystals by a reaction of SiO2 with zirconia in the matrix glass. Therefore, if the alkali metal component in the matrix glass elutes into the molten glass, zircon is likely to be formed in the refractory to increase the viscosity of the matrix glass, whereby cracks tend to be formed in the refractory.
Further, in recent years, attention has been drawn to an electric melting method of directly applying an electric current to a glass raw material to heat and melt it, as a method for producing glass of high quality while saving energy. In a case where the electric melting method is employed, the refractory is required to have a higher electrical resistivity than molten glass so that an electric current flows in the molten glass. In the case of the above-mentioned highly zirconia-based refractory containing an alkali metal component in the matrix glass, if it is attempted to directly apply an electric current to molten glass to heat and melt the glass, the alkali metal component present in such a refractory tends to show ionic conductivity, and further at a high temperature exceeding 1,000° C., zirconia also tends to show electrical conductivity, whereby a part of the applied electric power tends not to flow in molten glass but to flow in the refractory surrounding the molten glass, thus leading to a problem that this method cannot be applied.
In order to solve such a problem, Patent Document 1 proposes a highly zirconia-based refractory having a high electrical resistivity at 1,500° C. This refractory is made to have a composition which does not substantially contain a Na2O component which has a small ionic radius and which makes the electrical resistivity remarkably small, and instead, from 0.5 to 1.5% of B2O3 and at most 1.5% of K2O having a large ionic radius are incorporated to adjust the viscosity of the matrix glass thereby to obtain a highly zirconia-based refractory which has a high electrical resistivity and is free from cracking.
However, among the contained alkali metal and alkaline earth metal components (K2O, Rb2O, Cs2O, SrO and BaO), one or more are small in content at a level of at most 1.5%, and, hence, at a high temperature, zircon is likely to be formed in the refractory, such being inadequate to prevent cracking.
Patent Document 2 proposes a highly zirconia-based refractory having a high electric resistivity, which does not contain Na2O or K2O showing ionic conductivity, but instead, contains from 0.3 to 3% of at least one of BaO, SrO and CaO.
However, a problem has been pointed out such that cracking is likely to take place during heating one side, since neither Na2O nor K2O is contained.
Patent Document 3 proposes a highly zirconia-based refractory having a high electrical resistivity at a high temperature and excellent heat cycle resistance, as it contains K2O and Na2O in a total amount of from 0.01 to 0.12%, and contains K2O in an amount larger than Na2O.
However, since no alkaline earth metal component is contained, Na2O is incorporated, whereby the electrical resistivity has been inadequate.
Patent Document 4 also proposes a highly zirconia-based refractory having a high electrical resistivity at a high temperature. However, its electrical resistivity is inadequate, since it contains Na2O in an amount of at least 0.05%, and it also has a problem such that in contact with molten low alkali glass, Na2O is likely to elute, thus leading to cracking.
Patent Document 5 proposes to obtain a highly zirconia-based refractory having a high electrical resistivity by adjusting Al2O3 to be from 0.9 to 2.5%, SiO2 to be from 4.0 to 10.0%, ZrO2 to be from 86 to 95%, B2O3 to be from 0.1 to 1.2%, Na2O to be at most 0.04%, CaO to be at most 0.4%, Fe2O3 to be at most 0.1%, and TiO2 to be at most 0.25%.
However, in each of Examples disclosed, CaO is contained. CaO is a substance which is solid-solubilized in ZrO2, and it is reported that as solid-solubilized, it increases oxygen vacancies to bring about an oxygen ion conductivity. For this reason, the content of CaO is not suitable for the purpose of increasing the electrical resistivity at a high temperature.
Patent Document 6 proposes to limit Na2O to be less than 0.05 wt % and K2O to be from 0.01 to 0.2 wt % and further adjust components such as B2O3, Al2O3, BaO, CaO, Y2O3 and SrO, thereby to obtain a highly zirconia-based refractory having a high electrical resistivity with little change with time at a high temperature.
However, the refractory contains at least 0.01% of CaO which is solid-solubilized in ZrO2, and from 0.05 to 0.4% of Y2O3. For the above-mentioned reason, the content of CaO is undesirable from the viewpoint of the electrical resistivity. Y2O3 is also known to be solid-solubilized in ZrO2 thereby to increase oxygen vacancies and bring about an oxygen ion conductivity, and if it is contained, it likewise lowers the electrical resistivity.
Patent Document 7 proposes to add CrO3, Nb2O5, MoO3, Ta2O5 and WO3 thereby to obtain a highly zirconia-based refractory having a high electrical resistivity.
However, in each of Examples disclosed, neither K2O nor an alkaline earth metal component is contained. Further, more than 98.5% is required to be occupied by components of ZrO2+HfO2, SiO2, Al2O3, Y2O3, B2O3, CrO3, Nb2O5, MoO3, Ta2O5 and WO3, and, hence, it is not possible to add an alkali metal component, an alkaline earth metal component, P2O5, etc. in an amount of at least 1.5%, such being inadequate to prevent cracking during the temperature rise and operation.